15.12 : α-ハロカルボニル化合物の反応: 求核置換

α-halocarbonyl compounds undergo nucleophilic substitution at their α-position via an S_N2  pathway.

With α-halo ketones, the nucleophiles used must be less basic since strongly basic nucleophiles result in enolate formation.

On the contrary, α-halo acids react well with strongly basic nucleophiles via the S_N2 pathway.

Under basic condition, α-halo acids first ionize to its conjugate-base anion. Further deprotonation of the α hydrogen to form the dianion does not take place and instead, the anionic species undergoes a substitution reaction.

S_N1 reactions are unfavorable for α-halocarbonyl compounds, as they form less stable carbocations at the α-position.

The resonance structure of the intermediate carbocation involves electron-deficient oxygen, making it less resonance-stabilized.

Electrostatic interaction of the carbocation with the bond dipole of the carbonyl group also destabilizes the α-carbonyl carbocation.